1. Field Of The Invention
This invention relates to a system which mechanically polishes wafers used in the manufacture of semiconductor elements.
2. Prior Art
As semiconductor elements become increasingly smaller, for example VLSI technology the wiring technology associated with such devices requires smaller wiring pitches. Additionally a multitude of interconnect levels are present. As each wiring level is added during device fabrication, those coincident steps cause the surface topography to become increasingly severe. Wafers which have initially rough surfaces create difficulties with each succeeding processing, step such as photolithography, RIE etching, insulation and metalization. Thus, a standing requirement in the manufacture of semiconductor devices is to begin with wafers which have a high degree of planarization. One known technique is mechanical planarization however the tools which perform this step are manually loaded, require excessive setup time and the wafers must be reloaded into a brush cleaning tool following planarization. Thus an initial deficiency in the prior art is the lack of a system which has high throughput rates yet achieves a high degree of planarization on such wafers. One known wafer polishing tool is illustrated in FIG. 1. This tool mechanically polishes wafers by holding the wafer substrate against a rotating wheel. That is, a wafer 10 is manually placed in a wafer template 12 and positioned on the large polishing wheel 14. The template fits in a rotating holder 16 which in turn is held in place by an arm 18 to provide the necessary pressure against the wheel 14. A slurry is dispensed near the holder 16 as the wheel 14 and holder 16 rotate. FIG. 2 illustrates the directions of movement. As the action progresses, insulator is first removed from the projecting steps causing the topography to become planer. Uniform insulator removal is accomplished by adjusting holder rotation speed and pressure. A computer model may be used to interact the variables and establish the speed of holder 16 which will maximize uniformity for a given speed of polish wheel 14. Thus, as illustrated in FIG. 2, while the large polishing wheel 14 rotates in a counterclockwise direction, the smaller holder itself also rotates. In general, because the diameter of wafer holder 16 is less than the radius of polishing wheel 14, oscillatory motion of holder 16 between the edge and center of wheel 14 may be used to further improve the uniformity of material removal. The rotating holder 16 presses the wafer against the polish wheel 14 with a pressure in the range of 10 pounds per square inch. This prior art polishing apparatus has several deficiencies. As wafer diameter increases, the cost and size of such a conventional polishing tool increase dramatically. Moreover, since the wafer is being pressed against the polish wheel at a high pressure, any non-uniformity in either the rear surface of the wafer or the apparatus that contacts the rear surface of the wafer will produce non-uniform material removal at the polish surface. Finally, because material removal rate is proportional to the differential velocity between the Wafer and the polishing wheel, the wafer surface is subjected to a continuum of polish rates if the wafer is held stationary. This non-uniformity in polishing rates can be addressed by varying the wafer spin speed with respect to the speed of the rotating polish wheel. However, in theory the material removal rate can be made only 95% uniform for an 8 inch wafer being polished on a 22 inch polishing wheel.
Reference is made to IBM Technical Disclosure Bulletin, Vol. 21, No. 7, December 1978, p. 2733, "Controlled Wafer Backside Polishing" which discloses the concept of controlling the polish rate and thus polish profile by introducing discontinuities in the abrasive surface of the polish wheel.
Reference is made to U.S. Pat. Nos. 1,899,463; 2,536,444; 3,748,677: 3,907,471 and 4,256,535 which are representative of polishing devices which use one or more flat horizontally rotating polishing wheels. U.S. Pat. No. 1,899,463 employs upper and lower polishing rollers to simultaneously polish two sides of a workpiece U.S. Pat. No. 2,536,444 employs a series of opposed grinding drums to polish the surface of the strip material and U.S. Pat. No. 3,748,677 employs a rotating carrier for wafers to transport wafers in succession between two opposed rotating brushes.
In U.S. Pat. No. 1,899,463, the vertically rotating rollers are set mechanically parallel to each other. In the context of the 463 Patent polishing on both sides of the workpiece is achieved. The system is not satisfactory for single-sided polishing where a high degree of precision is required.